The present invention relates to a shift control algorithm for an automatic transmission.
Automatic transmissions commonly employ a xe2x80x9cclutch-to-clutchxe2x80x9d shift control strategy wherein the interchange between successive forward ratios is accomplished by the disengagement of one of the torque transmitting mechanisms and the substantially simultaneous engagement of another torque transmitting mechanism. To accomplish this strategy, pressure control devices such as pulse width modulated (PWM) solenoids are driven by a transmission controller to directly control oil pressure at the transmission torque transmitting mechanisms.
The PWM signal may be modulated at a constant frequency (e.g. 102 Hz) with varying duty cycles to change oil pressure at the torque transmitting mechanism. These pressure pulses generated by the inherent operation of the PWM solenoid may, in certain circumstances, result in torque spikes being transmitted from the transmission to the vehicle driveline. Such torque spikes may result in a poor shift feel or a shift noise or growl due to the excitement of the driveline. The noise may be accentuated where the transmission is installed in a light, stiff vehicle system. Further the noise may be especially noticeable when the transmission output shaft is stopped. The integration of a spring pack in the torque transmitting mechanism or the addition of an accumulator to the torque transmitting mechanisms feed circuit may reduce the effects of torque spikes but at the expense of increasing system content.
A means is needed to minimize the transfer of torque spikes originating in the transmission to the driveline where it may lead to dissatisfying customer shift noise or feel.
The present invention relates to a shift control algorithm for an automatic transmission. In particular, the algorithm operates to minimize torque spikes transmitted through the transmission by locking the output drive shaft through the application of torque transmitting mechanisms prior to a shift transition.
When such a shift is commanded, the algorithm verifies the vehicle is effectively stopped prior to applying an output shaft lock-up torque transmitting mechanism. Once the lock-up torque transmitting mechanism reaches capacity, the shift may proceed by either applying a torque transmitting mechanism to capacity or releasing it to below a threshold pressure. The lock-up torque transmitting mechanism is then released, completing the shift.
This shift control algorithm minimizes poor shift feel and noise. It may be easily employed in an existing transmission without the need for additional costly hardware and lead time.